1. Field of the Invention
The present invention relates to a liquid crystal display device of externally charging type and a hand-writable liquid crystal board set including the device. More specifically, the present invention relates to a hand-writable liquid crystal board set comprising: a liquid crystal display device of externally charging type in which the display or erasing of an image is carried out by varying the state of light scattering in the liquid crystal using electro-optical effects, or the provision or removal of static electric charges; display means to display an image on a board screen; erasing means for erasing the recorded image; and a power source for generating a surface potential on the board screen using the display means.
2. Description of the Related Art
Many proposals have already been made relating to liquid crystal display devices using electro-optical effects, such as the current effect, electric field effect, or heat effect of liquid crystals. Of these, a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No.(sho) 55-96922, comprises a pair of transparent conductive layers, and a liquid crystal-dispersed polymer layer sandwiched therebetween in which micro-capsules hermetically holding liquid crystal therein are scattered. Since this liquid crystal display device effects the display by varying the alignment of liquid crystal by turning on and off the application of an electric field between electrodes, it is possible to easily obtain a large-sized display screen with a simple structure. Nevertheless, this display device requires use of polarizing plates, thus exhibiting drawbacks such as a dark display image and a narrow field of view.
U.S. Pat. No. 4,435,047 as well as Japanese-translated PCT Patent Application Laid-Open No.(sho) 63-501512, discloses a liquid crystal display device which has a simple structure with no polarizing plate, thereby the above drawbacks are eliminated, and which facilitates the provision of a large-sized display screen. For this purpose the liquid crystal display device includes a pair of transparent electrodes, and a liquid crystal-dispersed polymer layer sandwiched between the electrodes in which liquid crystal is finely dispersed in the polymer matrix. The liquid crystal display device having such a structure, however, cannot be applied to a so called externally charging type liquid crystal display, in which recording and erasing can be effected from the outside surface of the display screen.
As an improvement in this respect, Japanese Patent Application Laid-open Nos.(sho) 61-83521 and (hei) 3-43715 disclose a liquid crystal display device comprising a liquid crystal-dispersed polymer layer in which liquid crystal is finely dispersed in the polymer matrix and a conductive layer is coated in a layer thereon; and an image display method in which a recording image is formed by providing static charges on the display screen from the outside of the liquid crystal-dispersed polymer layer (of the display screen) and thus changing the phase and/or alignment of the liquid crystal.
However, none of these liquid crystal display apparatus described heretofore would allow the hand-writing or erasure of images on the display screen from the outside of the liquid crystal display device.
A liquid crystal display apparatus disclosed in Japanese Patent Application Laying-Open No.(sho) 61-83521, performs display and erasing of the recorded image by providing or removing static charges in a form of corona charge flow from the outside of the display screen of the liquid crystal display device. The present inventors have investigated the liquid crystal display apparatus of this kind, and have found that the provision of electric charges to the display screen could cause the cholesteric liquid crystal to change its phase to form a recording image, but the removal of the static charges could not bring back the phase-changed cholesteric liquid crystal to the initial state, or could not erase the recorded image. In order to erase the recorded image, it was found that the liquid crystal must be heated above the phase transition temperature, or be further provided with an electrically conductive sheet through which an a.c. electric field can be applied. It was also found that, for repetitions of display, the remaining static charges must be removed to effect the next display. In sum, it was found that complicated operations were required to display and erase recorded images repeatedly.
In a liquid crystal display device disclosed in Japanese Patent Application Laid-Open No.(hei) 3-43715, if electric charges are provided to the display screen from the outside by any of various charging techniques, the liquid crystal is aligned and this changes the state of light scattering in the liquid crystal display device to display a recorded image. This disclosure teaches that, to erase the recorded image thus obtained, it is necessary to provide opposite electric charges equivalent to the amount of the static charges present in the area of the recorded image. In practice, however, it is very difficult to perform such an operation since the amount of the opposite charges may be slightly more or less than that of the static charges. This situation makes the erasing operation difficult. Moreover, the disclosed apparatus disadvantageously exhibits shortness of memory retention of the recorded image in 20.degree. C. air at a relative humidity of 90% or under an environment of high humidity.
Some hand-writable liquid crystal board sets have been manufactured utilizing the above described liquid crystal display apparatuses, but for the reasons described above the devices are unsatisfactory. In other words, the former apparatus needs complicated display and erasing processes to repeatedly display images and erase them, or is problematic in its handling and performances. For the latter apparatus, erasing of the recorded image is not feasible, the liquid crystal board is not durable, and its memory retention of the recorded images under a high humidity environment is not satisfactory. For these reasons, it has been impossible to obtain a hand-writable liquid crystal board having good operativity and sufficient durability and performance.